Wireless network technologies: WLAN, WPAN, ad hoc...
Transcript of Wireless network technologies: WLAN, WPAN, ad hoc...
Wireless network technologies: WLAN, WPAN, ad hoc networks
Prof. Fabio Martignon
Summary (1)
� Introduction to wireless networks� Main differences with wired networks
� Radio channel
� Mobility management
� WLAN� 802.11 standards
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� 802.11 standards
� Network architecture
� MAC (legacy and QoS MAC 802.11e)
Summary (2)
� WPAN� Bluetooth
� Network architecture
� Medium access control
� Network formation
� ZigBee
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� ZigBee� Network architecture
� Medium access control
� Ad Hoc
� Definitions and basic networking issues
� Routing schemes
Wireless Networks
� Is the transmission medium the only difference?
� The peculiar medium characteristics have
Wireless or wired, which is better?Wireless or wired, which is better?
Well, it depends on the situation!Well, it depends on the situation!
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characteristics have great impact on system characteristics
� Wireless networks allow users to move and naturally manage mobility
wiredwire-less
Network architecture
Backbone network
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Access network
Wireless access networks
� Wireless networks are mainly access networks
� Backbone networks composed of radio point-to-point links are usually not considered wireless networks
� Wireless access networks are more
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� Wireless access networks are more challenging and have many fundamental differences with respect to wired access networks
� The first main difference is that the transmission medium is broadcast
Broadcast channel
Centralized broadcast channel
Distributed broadcast channel
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Centralized broadcast channel
� Fixed access point (cellular systems, WLAN, WMAN)
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Wired network
� Mobile-access point connection
Centralized broadcast channel
� Cellular coverage: The territory coverage
is obtained by Base Stations–BS (or Access Points) that provide radio access to Mobile Stations–MS within a service area called CELL
Base
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BaseStation
MobileStation
Cell
Distributed broadcast channel
� Ad-hoc wireless networks (mesh networks, sensor networks)
� mobile - mobile connections
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Distributed broadcast channel
� In multi-hop operation mobile stations can forward information
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sourcedestination
relay
relay
Wired-Wireless networks:Main differences
� Shared transmission medium
� Multiple access mechanisms
� Radio resource reuse
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Centralswith
cable
Radiochannel
Wired-Wireless networks:Main differences
� Radio channel� Variable channel characteristics
� Advanced modulation and coding schemes
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� User mobility
� Stand-by mobility
� Active session (conversation) mobility
Wired-Wireless networks:Main differences
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Wired-Wireless networks:Main differences
In this course we’ll focus on wireless � In this course we’ll focus on wireless technologies (how these problems are solved in practice):
� But first a few comments on:
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� But first a few comments on:
� Wireless channels
Wireless channel
� Very bad channel compared to other wired mediums
� Signals propagation is subject to:
� High attenuation due to distance
Supplementary attenuation due to
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� Supplementary attenuation due to obstacles
� Multipath propagation
Wireless channel: radio spectrum
f
c=λ
m/s 103 8⋅=c
� Radio waves
� Wave length
� Light speed
� Frequency f
)2cos()( ϕπ += ftts
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Wireless channel: radio spectrum
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Wireless channel: radio spectrum
� Higher frequencies:
� more bandwidth
� less crowded spectrum
� but greater attenuation through walls
� Lower frequencies
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� Lower frequencies
� bandwidth limited
� longer antennas required
� greater antenna separation required
� several sources of man-made noise
Wireless channel: antennas
� Transmission and reception are achieved by means of an antenna
� An antenna is an electrical conductor or system of conductors� Transmission - radiates electromagnetic energy
into space� Reception - collects electromagnetic energy from
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� Reception - collects electromagnetic energy from space
� In two-way communication, the same antenna can be used for transmission and reception
� Isotropic antenna (idealized)� Radiates power equally in all directions (3D)� Real antennas always have directive effects
(vertically and/or horizontally)
Wireless channel: attenuation
� Isotropic radiator transmitting power PT uniformly in all directions
distance d
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� Power density F at distance d is:
]W/m[4
22d
PF T
π=
distance d
source area
Wireless channel: attenuation
� Directive characteristics of real antennas concentrate power in some directions
� This effect can be modeled using the gain g(θ) in the direction θ
direction in distanceat power )(
dg
θθ =
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� The maximum gain gT is conventionally in the direction θ =0.
24/)(
dPg
T πθ =
Wireless channel: attenuation
� The power density in the maximum gain direction is given by:
]W/m[4
)( 22d
gPdF TT
π=
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� The product PT gT is called EIRP (Effective Isotropically Radiated Power) and it is the power required to reach the same power density with an isotropic radiator
Wireless channel: attenuation
� The received power depends on the power density at the receiver antenna and its equivalent area:
� For an isotropic antenna we have:
� While for a directive antenna we can
eR AdFP )(=
πλ4
2
=eA
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� While for a directive antenna we can concentrate energy:
� where gR is the receiver antenna gain
� Therefore:
π4
eRR AgdFP )(=
2
4
=d
ggPP RTTR πλ
Wireless channel: free space model (Friis)
� The received power is ∝ d-2
� This is known as the free space propagation
22
44
=
=fd
cggP
dggPP RTTRTTR ππ
λ
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� This is known as the free space propagation model
� It can be used for example with point-to-point radio links
Wireless channel: propagation impairments
� Unfortunately, in real environments the propagation of electromagnetic waves is more complex than in free space:
� Reflection
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� Shadowing
Wireless channel: propagation impairments
� DiffractionWhen the surface encountered has sharp edges. Bending the wave
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� ScatteringWhen the wave encounters objects smaller than the wavelength (vegetation, clouds, street signs)
Wireless channel: two ray model
� Just in the case of reflection with only two rays (a direct ray and a reflected one) the received power is quite different w.r.t. free space
� It can shown that: 4
22
)(d
hhggPdP RT
RTTR ≅
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hR
hT
d
d
Wireless channel: empirical models
� More complex scenarios can hardly be modeled with closed formulas
� Empirical models are usually adopted where received power is ∝ d-η
ηπλ
dggPP RTTR
1
4
2
=
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� where η is the propagation factor which typically ranges between 2 and 5
� More complex empirical models (e.g. Hata) take into account several parameters including scenario (urban, rural), antenna heights, etc.
ηπ dggPP RTTR 4
=
Wireless channel: empirical models
� Okumura/Hata formula:
� where� f frequency in MHz (from 150 to 1500 MHz)
( ) [dB] loglog55.69.44
)(log82.13log16.2655.69
dh
hahfL
T
RTP
−++−−+=
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� hT base station height (in m)
� hR mobile station height (in m) – a(hR) correlation factor depending on area shape
� d distance (in km)
� For a 900 MHz system, hT = 30 m, a(hR) ≈ 0:
dL P log22.3542.126 +=
Wireless channel: multipath fading
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� Copies of the same signal arrive from different paths
� Their combination at the receiver depends on:
� Number of copies
� Relative shift
� Amplitude
� Frequency
Wireless channel:multipath fading
� The resulting signal can be attenuated …
-1,5
-1
-0,5
0
0,5
1
1,5
0 5 10 15 20
s(t)
s(t+T)
s(t)+s(t+T)
T=4/5π
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-2,5
-2
-1,5
-1
-0,5
0
0,5
1
1,5
2
2,5
0 5 10 15 20
s(t)
s(t+T)
s(t)+s(t+T)
-1,5
� … or even amplified !!!
T=4/5π
T= π /6
Wireless channel: shadowing
� Signal can be partially absorbed or reflected by obstacles
� Further attenuation called shadowing
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Part 1
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Part 1WLAN
� Wireless networks are standardized by IEEE (Institute of Electrical and Electronics Engineers)
� under 802 LAN/MAN standards committee.
http://grouper.ieee.org/groups/802/11/
Standardization of Wireless Networks
Application
PresentationISO-OSI
Session
Transport
Network
Data Link
Physical
ISO-OSI7-layermodel
Logical Link Control
Medium Access (MAC)
Physical (PHY)
IEEE 802standards
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Wireless Evolution of LAN
� 802.11-legacy standard first ratified in July 1997
� 802.3 LAN emulation
� 3 PHY’s were specified for 1 & 2 Mbps (FHSS, DSSS, Infrared)
� Two High Rate PHY’s ratified in Sep 1999
802.11 History
� 802.11a 6 to 54Mbps in the 5GHz band (OFDM)
� 802.11b 5.5 and 11Mbps in the 2.4GHz band
� PHY up to 54 Mbit/s in 2.4 GHz band ratified in 2003
� 802.11g
� 802.11a compatible with European spectrum regulation in 5GHz band ratified in 2004
� 802.11h (Transmit Power control to avoid interference with satellites, radars, etc …)
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� Enhanced security framework ratified in 2004
� 802.11i
� Enhanced MAC with QoS support ratified in 2005
� 802.11e
� High rate standard (PHY and also MAC) ratified in 2009 (draft 2007 with WiFi pre-standard products)
802.11 History
2009 (draft 2007 with WiFi pre-standard products)
� 802.11n
And more to come:� 802.11s: mesh networking
� 802.11p: WAVE—Wireless Access for the Vehicular Environment
� 802.11v: Wireless network management
� 802.11ac: Very High Throughput (0.5 – 1 Gbit/s)
� 802.11af: White-Fi (CRN, using TV Whitespace …)
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� WLAN Timeline
802.11 History
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860 Kbps
900 MHz
1 and 2 Mbps
2.4 GHz
Proprietary
� 802.11 � 802.11a,b
� 802.11g
1988 1990 1992 1994 1996 1998 2000 2002
2.4 GHz
11 and 54 Mbps Up to 600 Mbps
Standards-based
5 GHz
� IEEE 802.11Begins Drafting
2004 2008 2010
� 802.11n � 802.11i
� 802.11e
Wi-Fi
� Wi-Fi™ Alliance� Wireless Fidelity Alliance
� 500+ members
� Over 350 products certified
Wi-Fi’s™ Mission� Wi-Fi’s™ Mission� Certify interoperability of WLAN products
(802.11)
� Wi-Fi™ is the “stamp of approval”
� Promote Wi-Fi™ as the global standard
� The standard provides two modes of operation which are:
� DCF (mandatory) – best effort contention service – uses CSMA with Collision Avoidance (CSMA/CA)
� PCF (optional) – base station
802.11 Protocol Architecture
� PCF (optional) – base station controls access to the medium – uses a polling mechanism with higher priority access to the medium, plus three types of frames: data, control and management
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(PCF)
WLAN:Network architecture
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Network architecture
Components
� Station (STA)
� Access Point (AP)� bridging wired/wireless
� BSS - Basic Service Set
� Infrastructure BSS: infrastructure based
� Independent BSS (IBSS): ad hoc
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� Independent BSS (IBSS): ad hoc
� ESS - Extended Service Set
� Set of Infrastructure BSS
� A set of access points connected through a:
� DS – Distribution System (not directly addressed in the standard)� Wired
� Wireless (WDS)
Basic Service Set (BSS)
� Set of stations (STAs) controlled by the same “Coordination Function” (logic function that manages the access to the shared channel)
� Similar to the concept of “cell” in � Similar to the concept of “cell” in mobile radio networks
� Two types of BSS:� Infrastructure BSS
� Independent BSS (IBSS)
Infrastructure BSS
Centralized architecture
BSS
Independent Basic Service Set (IBSS)
Ad hoc architecture
IBSS
Extended Service Set (ESS)
BSS
• Wired DS• Layer 2 connection
among BSSs
BSS
BSS
Extended Service Set (ESS)
BSS
Wireless Distribution System(WDS)
BSS
BSS
Distribution System
CBA
R
AP2AP1
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� STA associates to one, and only one, AP through an association procedure
� Association procedure is “equivalent” to plugging the ethernet cable
� An ESS is a layer-2 network, and therefore a single IP subnet with its own addressing space
CBA
Distribution System
Bridge
DS
AP
StationA
StationB
StationC
STA
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� APs behave like an Ethernet bridge (layer-2 switch)
� Association tables are used for the bridging process
� For example, frames received via the DS and indicating as destination a wireless STA are forwarded to the wireless interface once converted to 802.11 format
A B CSTA
Distribution System
CBA
R
AP2AP1
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� Question: How exactly can an IP packet go from router R to STA B (assume ARP tables empty)?
CBA
Distribution System
A
R
AP2AP1
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� What happens when a station moves from AP1 to AP2? (handover or handoff)
AA
WLAN:Medium Access Control
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Medium Access Control
MAC services and functionalities
� Channel access
� Error recovery
� Fragmentation and reassembly
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� Fragmentation and reassembly
� Power saving
� Addressing
� Framing
Channel access
� The access to the transmission medium is regulated by the so called “coordination functions”
� Two coordination functions are defined
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defined� Distributed Coordination Function (DCF)
� Based on CSMA with backoff
� Point Coordination Function (PCF)� Collision free access based on polling
� Several BUGS in the standard – never implemented in commercial devices
Error recovery
� Absolutely necessary on a “noisy” (wireless) channel
� Only for unicast transmissions (broadcast service is unreliable) Frame
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service is unreliable)
� Based on a positive acknowledgment per received frame (“stop & wait”)
� Requires to use retransmission timers
ACK
Question: Do we have ACKs in Ethernet? Why?
Interframe spacing
Previous Frame
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� Several time intervals regulate the channel access
� These are minimum waiting intervals after last and are based on the physical carrier sensing mechanism
Interframe spacing
� Short Inter Frame Spacing (SIFS):
� High priority transmissions can start after a SIFS after previous transmission
� PCF Inter Frame Spacing (PIFS):
� Interframe space used to issue polling frames in PCF mode
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PCF mode
� DCF Inter Frame Spacing (DIFS):
� Regular data transmission in DCF mode can start after a DIFS
� Extended Inter Frame Spacing (EIFS):
� Used in special cases when previous transmission cannot be decoded
Distributed Coordination Function
� DCF allows the coordination among stations without the need of a central controlling entity
� It can be used either in a IBSS and in a infrastructure BSS
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infrastructure BSS
� It is based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
� Before starting a transmission, the station listens to the channel:
� If the Channel is idle (for at least a DIFS): start transmitting
� If the Channel is busy: wait and start backoff
Distributed Coordination Function
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Collision Avoidance through the Backoff procedure
� Before transmitting, a station waits for a time period equal to a random number of slots (backoff) + DIFS
� The random number is uniform between 0 and CW (Contention Window)
� If during backoff the channel becomes busy, the counting down of slots is stopped, and it is then resumed when the channel is idle again
� If consecutive packets need to be transmitted, the backoff procedure is used between pcks also when channel is idle
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�
procedure is used between pcks also when channel is idleDIFS DIFS
backoff
backoff Remaining backoff
Station A
Station B
Station C
Backoff Mechanism –the CW parameter
� backoff is uniform in [0, CW]
� CW is set dynamically:� If an error/collision
occurs, (almost)
511
1023
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� If an error/collision occurs, (almost) double CW (up to CWmax=1023 slots)
� If transmission is correct, set CW=CWmin=31
63127
255
Initial attempt1st retransmission
2nd retransmission3rd retransmission
4th retransmission5th retransmission
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Error recovery in DCF
� Transmitting station can recover corrupted frames through retransmission
� Transmission feedback from receiver is based on a “positive acknowledgement”� Reception of unicast frames needs to be
acknowledged
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acknowledged
� If acknowledgement is not received, the frame is considered lost and retransmitted
� There is a maximum number of retransmissions per frame
� Retry Counters� Short Retry counter (for “short” frames)
� Long Retry counter (for “long” frames)
Transmission example
Ack
DataSrc
DestContention Window
DIFS
SIFS
DIFS
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� SIFS < DIFS, therefore ACKs have priority over data frames
Next MPDUOthers
Contention Window
Defer Access Backoff after Defer
DIFS
Hidden Terminal
A
BC
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� Station A is “hidden” to station C
� Since B is in range of both A and C, a collision occurs (i.e., two (or more) signals arrive at
the same time in the same place, namely station B)
collision
Solution to the Hidden Terminal
� The standard adds a procedure of logical (virtual) carrier sensing to the physical one
� Control frames are used, in which it is coded the so called Network Allocation Vector (NAV)
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Vector (NAV)
� The NAV contains the duration of the communication which is currently occupying the channel
� Mobile stations that receive such control frames do not try to access the channel during the time specified in the NAV
Virtual Carrier Sense
source
destination
RTS
DIFS
CTS
SIFS SIFS
DATA
SIFS
ACK
NAV (RTS)
source
destination
RTS
DIFS
CTS
SIFS SIFS
DATA
SIFS
ACK
NAV (RTS)
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� Ingredients
� Control frames:
� Request To Send (RTS)
� Clear To Send (CTS)
� Network Allocation Vector (NAV)
neighbors
NAV (RTS)
NAV (CTS) Random Backoffneighbors
NAV (RTS)
NAV (CTS) Random Backoff
Example
Station 2 talks to Station 1
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Hidden terminal solved
RTS CA
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CTSCTS
B
Exposed terminal
� … but another problem is created
AB
CTS
CTS
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B
C
D
RTS
Here, D cannot send data to C, even though the transmission from A will not interfere (it will not even reach C), and vice-versa (the transmission from D will never reach B)
Overhead
� The exchange of control frames reduceschannel capacity
� Transmission efficiency depends on:� Channel quality
� Data frame size
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� Data frame size
� 802.11 standard defines a threshold (RTSThreshold) on the size (D) of data frames:� If D < RTSThreshold, RTS/CTS exchange is NOT
used
� If D > RTSThreshold, RTS/CTS exchange is used
Throughput analysis of CSMA
� Simplified model� No hidden terminals� CSMA: like Aloha but transmit only when
you sense the channel freeMaximum delay (distance) between 2 stations
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ττ
a=τ/T
Frame duration (supposed fixed)
Throughput analysis of CSMA
� On the channel we have cycles of Busy (at least one station senses the channel as busy) and Idle (all stations sense the channel free) periods
� The throughput S can be given by:
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� The throughput S can be given by:
IBS
+= α
� where B and I are the average busy and idle periods, and α is the probability that there is a successful transmission in a busy period
Throughput analysis of CSMA
� Making the same assumptions of the Aloha infinite population model we have:
1I
e aG
=
= −α
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)1)(1()1(
1
ZaeaeB
GI
aGaG ++−++=
=
−−
� where Z is the time when colliding transmissions partially overlap
Throughput analysis of CSMA
� It can be shown that:
� Therefore we get:
Ge
aeaZ
aG
aG 1
1−
−+= −
−
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� Therefore we get:
aG
aG
eaG
GeS −
−
++=
)21(
Throughput analysis of CSMA
When a is small, all stations are very close to each other (the feedback from channel listening is almost immediate)
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Throughput analysis of CSMA/CA
� Similarly to the general model we can derive a model for 802.11 DCF
1I
e aG
=
= −α a = interframe spaceb = duration of RTS and CTS
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))(1()231(
1
ZabebaeB
GI
aGaG ++−+++=
=
−−
aGaGaG
aG
GebaebGeaG
GeS −−−
−
++−−−++=
)22()1)(1()21(
Point Coordination Function (PCF)
NAV
BeaconPoll ST1
Frame from ST1CF ack
Poll ST 2ack ST 1
CFend
Frame from ST1CF ack
SIFS
SIFS
SIFS
SIFS
SIFS
PC
ST
Other ST
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� PCF has never been used in commercial devices
� PCF cannot guarantee QoS due to a couple of “mistakes”
CFP
Point Coordination Function (PCF)
frame
ACK
frame
ACK
Beacon
Expectedf start of CFP
Actual start
SIFS
DIFS
SIFS
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� Unpredictable inter-beacon interval
� No constraint on frame transmission duration
CFPMaxDuration
SIFS SIFS
802.11e
� Flow differentiation
� 4 queues for 4 traffic classes
� Transmission Opportunities (TXOP)
� Defined as the Maximum transmission time per transmission
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� Direct transmission
� Block ACK (single ACK for a “train” of frames)
� Hybrid Coordination Function (HCF) with two access modes
� Contention based (EDCA, Enhanced Distributed Channel Access)
� Contention free (HCCA, HCF Controlled Channel Access)
EDCA
� 4 Access Categories (AC)
� AC_VO: voice
� AC_VI: video
� AC_BE: best effort
� AC_BK: background
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� Each AC is characterized by different backoff parameters:
� AIFS[AC]: inter-frame space
� CWMin[AC]: minimum backoff window
� CWMax[AC]: maximum backoff window
� TXOPlimit[AC]: maximum transmission duration
Example: access with EDCA
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802.11eEach queue behaves as a single DCF contending entity
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� Several backoff entities inside the same station (Virtual collision handler)
HCCA
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� Contention-free and contention-based transmissions can be mixed together
Frame Format
FrameControl
DurationID Addr 1 Addr 2 Addr 3 Addr 4Sequence
Control CRCFrameBody
2 2 6 6 6 62 0-2312 4
802.11 MAC Header
Bytes:
used to set NAV
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ProtocolVersion
Type SubTypeToDS
RetryPwrMgt
MoreData
WEP Rsvd
Frame Control Field
Bits: 2 2 4 1 1 1 1 1 1 1 1
DSFrom More
Frag
any retransmitted frame sets the Retry bit to ”1”.
”1” means devices are in power-saving mode, ”0” active. (APs are not allowed to power-save mode because they perform management functions).
set to ”1” when frame encrypted
Type & Subtype
� Data (type=10)
� Control (type=01)
� Management (type=00)
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Subtypebit
Message
0000 Association request
1000 Beacon
1011 Authentication
Subtypebit
Message
1011 RTS
1100 CTS
1101 ACK
Subtypebit
Message
0000 DATA
0001 DATA+CF ack
0010 Data+CF poll
Management Control Data
Addresses
� Four address fields: 48 bits long (as in Ethernet). Not always used, and not for all the MAC frames: they depend on frame type
� Different types of addresses:
� Destination Address (DA): final destination
Source Address (SA): frame source
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� Source Address (SA): frame source
� Receiver Address (RA): receiving station
� Transmitter Address (TA): transmitting station
� Basic Service Set ID (BSSID): BSS address
� Infrastructure BSS: MAC address of the AP
� IBSS: random number or address of one station
Addresses
� Rule of thumb:
� address1 used for the receiver;
� address2 used for the transmitter;
� address3 used for filtering by the receiver (frames discarded from a BSS other than the associated one).
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the associated one).
Network Management
� Management frames are used for:
� Scanning
� Authentication
� Association
� Power Management
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� Power Management
� Synchronization
Wireless Mesh Networks
Mesh Point
Mesh Portal
Mesh Portal
InternetInternet
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Point
Mesh Point
Mesh Point
Portal
Mesh AP
Mesh AP
STASTA STA STA STA
Mesh Network
BSSBSS
Part B
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Part BWPAN
Personal Area
WLANOn-campus: Office,
School, Airport, Hotel, Home
WPAN
Person Space: Office, Room, Briefcase, Pocket, Car
Short Range/Low Power
Voice AND Data
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CellularOff-Campus Global Coverage
Low-cost
Small form factor
Many Co-located Nets
Universal Bridge
Personal Area
Cable Cable
Landline
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Personal AdPersonal Ad--hoc hoc ConnectivityConnectivity
Cable Cable ReplacementReplacement
Data/Voice Data/Voice Access PointsAccess Points
IEEE 802.15
Wireless Personal Area Networks (WPANsTM)� Short range
� Low power
� Low cost
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� Low cost
� Small networks (including point-to-point links)
� “Personal Operating Space”
� Working Group (WG) created by IEEE, pushed by Bluetooth success
IEEE 802.15 Wireless Personal Area Network (WPAN) Working Group
IEEE Wireless Standards
Task Group 1WPAN/Bluetooth™
Task Group 2Coexistence
IEEE 802.11WLAN Working Group
IEEE 802.16WMAN Working Group
IEEE 802.18
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WPAN/Bluetooth™ Coexistence
Task Group 3WPAN High Rate
Task Group 3aWPAN Alt.
Higher Rate
Task Group 4WPAN Low Rate
IEEE 802.20Mobile BWA Working Group
IEEE 802.18Radio Regulatory TAG
IEEE 802.19Coexistence TAG
IEEE 802.22Wireless Regional Area
Networks
But very long life battery!
802.15 family
Chart Title
802.15
802.2 LLC
{
{Service Specific
Convergence Sublayer(SSCS)
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= Draft in process or complete
= Draft not defined e.g., CFP, etc.
1 Mb/s
2.4 GHzWPAN-Bluetooth
Bluetooth(TM)802.15.1
MAC Sublayer802.15.1
11 Mb/s22 Mb/s55 Mb/s
2.4 GHzWPAN-HRHigh Rate802.15.3
110 Mb/s? Mb/s
?WPAN-HR
Higher Rate802.15.3a
MAC Sublayer802.15.3
2 kb/s20 kb/s
868-868.6 MHzWPAN-LRLow Rate802.15.4
2 kb/s20 kb/s
902-928 MHzWPAN-LRLow Rate802.15.4
2 kb/s250 kb/s
2400-2483.5 MHzWPAN-LRLow Rate802.15.4
MAC Sublayer802.15.4
PhysicalLayer{
{
= Other LLC
Bluetooth
Bluetooth vs. 802.15.1
� Bluetooth is an industrial standard for WPAN
� WG 802.15.1 adopted Bluetooth specifications for levels 1 and 2
� ’96-’97: Ericsson internal project
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� ’96-’97: Ericsson internal project
� ’98: Bluetooth Special Interest Group (SIG) (Ericsson, IBM, Intel, Toshiba, Nokia)
� ’99: Other companies join the SIG (3Com, Lucent Technologies, Microsoft, Motorola)
Bluetooth
� Radio technology
� Low cost
� Short range (10-20 m)
� Low complexity
� Small size
� Transmission band ISM 2.4
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� Transmission band ISM 2.4 GHz
� Only the first two levels are
standardized by IEEE 802.15.1
�� Danish king of the Danish king of the middle ages, Harald middle ages, Harald Blaatand II, aliasBlaatand II, aliasBluetooth Bluetooth (940(940--981)981)
�� Unified Denmark and Unified Denmark and SwedenSweden
Application scenarios
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� Headset
Application scenarios
100
� Data sinchronization
Application scenarios
Adsl, fiber, etc.
GPRS, UMTS, etc.
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� Access point
Physical layer
� ISM Band at 2.4 GHz
� 79 (only 23 in France and Japan) channels spaced by 1 MHz (2402-2480 MHz)
� G-FSK (Gaussian FSK) Modulation (1 Mb/s)� a binary one is represented by a positive frequency deviation
and a binary zero by a negative frequency deviation
102
and a binary zero by a negative frequency deviation
� Device classes
ClassPower(mW)
Power(dBm)
Range(Approximated)
Class 1 100 mW 20 dBm ~ 100 meters
Class 2 2,5 mW 4 dBm ~ 10 meters
Class 3 1 mW 0 dBm ~ 1 meter
Physical layer
� Frequency Hopping (FH)
� 1600 hops/s (625 µs per hop)
� FH sequence is pseudo random and defined by the clock and the address of the master station that regulates
103
of the master station that regulates channel access
� The other devices are slave and follow the sequence fk defined by the master
Physical layer
master
slave
fk fk+1 fk+2 fk+3
104
slave
625 µs
Physical layer
master
slave
fk fk+3 fk+4 fk+5 fk+6
3-slot packet
105
� Packets can span over 1, 3 or 5
slots (625 µs)
slave
625 µs
Physical layer
master
slave
fkfk+5 fk+6
5-slot packet
106
� Packets can span over 1, 3 o 5 slots
slave
625 µs
Piconet
� Piconet is the simplest architecture of a Bluetooth network
� A Piconet is an ad hoc network composed of 2 or more devices
� One device acts as master, and all the other(s) as slave(s)
107
other(s) as slave(s)� The communication is only between master
and slaves: slaves cannot communicate directly among themselves
� Up to 7 slaves can be activated� The other nodes can be in:
� Stand-by (not members of the piconet)� Parked (still members of the piconet, but not
active; up to 256 parked slaves)
Piconet
M
S
PSB
S
S
S
S
P
SB
SB
M= MasterS=Slave
SB=Stand-byP=Parked
108
� Addresses� MAC address 48 bits
� AMA (Active Member Address) 3 bits
� PMA (Parked Member Address) 8 bits
S
S S
S
P
Up to 8
Up to 256
Connection types
� Bluetooth considers two types of connections
� SCO (Synchronous Connection Oriented)� Symmetric� Bidirectional fixed-capacity connection (“circuit”)� Optionally FEC
Basic speed 64 kbit/s
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� Basic speed 64 kbit/s
� ACL (Asynchronous ConnectionLess)� Packet service between master and slaves based
on a polling mechanism� Several packet formats available� Rate up to 433.9 kbit/s symmetric (using 5 slot
packets in both directions) and 723.2/57.6 kbit/s asymmetric (using 5 slots in one direction and 1 slot packet in the other)
Multiple access
Master
Slave 1
110
Slave 2
Slave 3
SCO (Synchronous Connection Oriented)
ACL (Asynchronous ConnectionLess)
Protocol architecture
� Protocol stack non compliant with OSI model (adapted by 802.15.1 specifications with some compromise)
Other TCS RFCOMM SDP
Applications
111
some compromise)
� RF + Baseband equivalent to PHY + MAC
� Control plane for network creation and connection managementRF
Baseband
AudioLink Manager
L2CAP
Data
Protocol architectureAllows a device to discover services offered by other devices, and their associated parameters
112
Packet Format
� BT includes three parts:
Access code Header Payload
72 54 0-2745
113
� BT includes three parts:� An Access Code used for synchronization and
piconet identification
� An Header used for Link Control (LC) and ARQ
� A Payload whose format depends on the connection type and packet type (number of slots, FEC, etc.)
Packet Format: Access Code
� Access code:� There are three types of access codes:
Synchronization word
4 64 4
Preamble Trailer
114
� There are three types of access codes:
� Channel Access Code (CAC): used to identify the piconet. It defines a piconet and is used in all transmissions; it is based on the master MAC address
� Device Access Code (DAC): used for paging a device before network formation; it is derived directly from the device MAC address
� Inquiry Access Code (IAC): used to search for all BT devices in range (inquiry)
Packet Format: Header
� Header:� Active Member Address (AMA)
Type
3 4 8
AMA HECFlow ARQ SQN
1 1 1 X 3FEC coderate 1/3
115
� Active Member Address (AMA)
� Type: Packet type (there are 16 different types of packets)
� Flow: Flow control
� ARQ: Retransmission
� SQN: Sequence number
� HEC: checksum
Packet Format
Access code Header FHS payload
72 54 240 (2/3 FEC)
FHS
Access code Header ACL payload
72 54 0-2744 ([1,2,3]/3 FEC)
ACL
Frequency Hopping Synchronization, a special control packet
116
Access code Header SCO payload
72 54 0-2744 ([1,2,3]/3 FEC)
SCO
Access code Header SCO payload
72 54 80
DV ACL payload
32-150 (2/3 FEC)Synchronous Connection Oriented
Asynchronous ConnectionLess
Data Voice. A SCO data packet type for data and voice
Link controller: ARQ
Master
Slave A
A1
MA1
B1 A1 B1
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Slave B
NACK
ACK
MB1 MB1
Link controller: states
� Stand-by: device is disconnetted and the radio is off
� Connection: device is connected with other devices
� Inquiry: device is searching for other devices in range
118
devices in range� Inquiry Scan: device is idle, but it listens
to possible inquiry messages for short intervals of time (low duty cycle)
� Page: device is trying to connect to a specific device
� Page Scan: Similar to inquiry scan but for page messages
Link controller: states
Stand-by
Page InquiryPage Inquiry
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Connection
Page Inquiryscan scan
Master response
Slave response
Inquiry response
Page procedure
� If a device wants to connect to another device, it needs to know its address and then start the page procedure
� From the address, the Device Access Code (DAC) can be derived
� A stand-by device enters periodically in the page scan state and listens to receive its
120
� A stand-by device enters periodically in the page scan state and listens to receive its DAC
� Due to ISM band rules, page procedure cannot be performed on a fixed channel
� The device in page scan follows a pseudo random sequence tuning on 32 channels
Page procedure
� In order to limit the energy consumption, the device performs the page scan for 10 ms on a channel and then goes to sleep for a few seconds (from 1.28 to 3.85 s)
� At every scan, a different channel is used according to the scanning pseudo-random
121
according to the scanning pseudo-random sequence
� The paging device can calculate the sequence, but usually it doesn’t know the clock (phase)
� So it transmits the DAC on all possible frequencies sequentially
Page procedurefk fk+1 fk+2 fk+3 FHS
122
� In 10 ms, the paging device can transmit the DAC on 16 out of 32 channels
� The transmission sequence is repeated until a reply is received
� If after a sleep time no reply is received, the other 16 channels are considered
625 µs
Page procedure
� The reply message is actually the same DAC
� The connection is established in 2 sleep times in the worst case
� The paging device replies with a FHS packet including all information on the device, including address and clock
123
including address and clock
� Connection is established
� The paging device becomes the Master, and the scanning device becomes the Slave
M S
� What about energy consumption?
� Why the paging device continues to transmit the DAC, spending a lot of energy, while the scanning device just listen to channels every once and
Page procedure
just listen to channels every once and a while?
� Would a more fair approach be preferable?
124
Inquiry procedure
� The inquiry procedure allows to discover all the other active devices in range
� It is similar to the page procedure, but the Inquiry Access Code (IAC) is adopted instead of the DAC
� Also the inquiry scan sequence is pseudo random
125
� Also the inquiry scan sequence is pseudo random
� The reply is a FHS packet� Collisions may occur due to multiple devices
in range� After an inquiry, the devices switch to scan
in order to setup the connection� However, since it knows the clock of the
device, the scan time can be minimized
Low power modes
� When connected, the device can optionally enter low power modes
� Hold: slave stops listening to the channel for a time period agreed with the master (it keeps its AMA, Active Member Address)
� Sniff: slave alternates listening and sleep periods according to a cycle agreed with the
126
� Sniff: slave alternates listening and sleep periods according to a cycle agreed with the master (also here it keeps its AMA)
� Park: in this state, the slave releases its AMA and gets a PMA (Parked Member Address) from the master. It listens to the channel with a low duty cycle for receiving unpark message from the master
Protocols: Link Management
� The link management protocol is in charge of the connection setup, security and control procedures
� Setup of ACL and SCO links
� Management of security procedure
127
� Management of security procedure
� Add and remove slaves from a piconet
� LMP messages have priority over all the others
Protocols: Security
128
Protocols: L2CAP
� Logical Link Control and Adaptation Protocol (L2CAP)
� Adaptation functions (segmentation and reassembly) and multiplexing
AudioSDP RFCOMM TCS
129
BasebandACL SCO
L2CAPLMP Voice
AudioSDP RFCOMM TCS
Profiles
� Profiles are sets of standard functions that allow interoperability of devices of different vendors through a standard implementation of specific applications
� Object Push Profile� File Transfer Profile
� Generic Access Profile� Service Discovery
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� File Transfer Profile� Synchronization Profile� Advanced Audio Distribution� PAN� Audio Video Remote Control� Basic Printing� Basic Imaging� Extended Service Discovery� Generic Audio Video Distribution� Hands Free� Hardcopy Cable Replacement
� Service Discovery Application Profile
� Cordless Telephony Profile� Intercom Profile� Serial Port Profile� Headset Profile� Dial-up Networking Profile� Fax Profile� LAN Access Profile� Generic Object Exchange
Profile
Scatternet
M
S
S
S
S
S
S
M
M
131
� Devices can participate to different picones simultaneously
� A device can be Master only in one piconet (Why?)
� Devices can switch from one piconet to another using the hold and sniff modes
� Scatternet formation and routing are out of standard specifications
S
S
SSS
Scatternet (2)
M
S M/S
132
� Scatternet allows, when necessary, to create direct links
S
S
S
Zigbee
Low Rate - WPAN
� Most applications in Wireless Networking require high transmission rates
� Starting in the 90’s, a lot of efforts have been devoted to these application and to high rate wireless technologies: WLAN (IEEE 802.11), BlueTooth (IEEE 802.15), WiMax (IEEE
134
BlueTooth (IEEE 802.15), WiMax (IEEE 802.16)
� However, there are also several applications that require short range, low energy consumption and low rate
� Low Rate WPAN (LR-WPAN) have been considered for this specific application segment
Po
wer
Co
nsu
mp
tion
Co
mp
lexi
ty
802.11g
802.16(WiMax)
Low Rate - WPAN
135
Data Rate
802.15.4(Zigbee)
802.15.1(Bluetooth)
WPAN
802.11802.11b
Characteristics
� Low cost of the hardware (< 1$) and the software
� Low transmission range (~10-30m)
� Low latency, if necessary
136
� Low latency, if necessary
� And, above all, low energy consumption!
Applications
CONSUMER ELECTRONICS
TVVCRDVD/CDremoteBUILDING
AUTOMATION
securityHVACAMR
lighting controlaccess control
patient
137
ZigBee
RESIDENTIAL/LIGHT
COMMERCIAL CONTROL
securityHVAClighting controlaccess controllawn & garden irrigation
PC & PERIPHERALS
INDUSTRIALCONTROL
asset mgtprocess controlenvironmental
energy mgt
PERSONAL HEALTH CARE
mousekeyboardjoystick
patient monitoring
fitness monitoring
Toward Zigbee ...
� Starting in mid 90s, several manufacturers designed proprietary solutions for sensor networks …
� … with obvious compatibility and high cost problems
� A standardization activity was necessary: the
138
� A standardization activity was necessary: the Working Group 4 is created within the IEEE 802.15 project (2001)
� IEEE 802.15.4 standard, considering physical and MAC layers, is published in May 2003
� The technology takes the commercial name of
Zigbee: protocol stack
NETWORK LAYERStar/Cluster/Mesh
APPLICATION INTERFACE
APPLICATIONS Customer
ZigBee Alliance
SECURITY
139
PHY LAYER2.4 GHz 915MHz 868 MHz
MAC LAYERMAC LAYER
Star/Cluster/Mesh
SiliconApplication ZigBee Stack
IEEE802.15.4
Alliance
Zigbee: protocol stack
API UDP
ZA1 ZA2 … ZAn IA1 IAn
Application interface designed usinggeneral profile
End developer applications, designed using application profiles
140
IEEE 802.15.4 PHY
IEEE 802.15.4 MAC (CPS)
ZigBee NWK
MAC (SSCS)802.2 LLC
IP
Transmission & reception on the physical radio channel
Channel access, PAN maintenance, reliable data transport
Topology management, MAC management, routing, discovery
protocol, security management
general profile
Zigbee: channels and rates
BAND COVERAGE DATA RATE # OF CHANNEL(S)
2.4 GHz ISM Worldwide 250 kbps 16
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2.4 GHz ISM Worldwide 250 kbps 16
868 MHz Europe 20 kbps 1
915 MHz ISM Americas 40 kbps 10
ZigBee� DSSS- 11 chips/
symbol
� 62.5 K symbols/s
� 4 Bits/ symbol
Bluetooth� FHSS
� 1 M Symbol / second
� Peak Information Rate
ZigBee vs. Bluetooth
142
� 4 Bits/ symbol
� Peak Information Rate
~128 Kbit/second
� Peak Information Rate
~720 Kbit / second
ZigBee vs. Bluetooth
ZigBee:ZigBee:ZigBee:ZigBee:• Network join time = 30ms typically • Sleeping slave changing to active = 15ms typically• Active slave channel access time = 15ms typically
143
Bluetooth:Bluetooth:Bluetooth:Bluetooth:• Network join time = >3s• Sleeping slave changing to active = 3s typically• Active slave channel access time = 2ms typically
ZigBee vs. Bluetooth
Bluetooth ZigBee
AIR INTERFACE FHSS DSSS
PROTOCOL STACK 250 kb 28 kb
BATTERY rechargeable non-rechargeable
144
BATTERY rechargeable non-rechargeable
DEVICES/NETWORK 8 255
LINK RATE 1 Mbps 250 kbps
RANGE ~10 meters (w/o pa) ~30 meters
Zigbee: devices
Standard defines tow device types:
� Full Function Device (FFD):� Can transmit beacon frames� Can directly communicate with other FFD � Can make routing
145
� Can make routing� Can act as PAN coordinator
� Reduced Function Device (RFD):� Cannot route traffic� Cannot communicate directly with other RFD� Can only communicate directly with one FFD
Zigbee: topologies
Three topologies are defined:
1 - STAR TOPOLOGY
146
PAN Coordinator
Full Function Device
Reduced Function Device
2 - MESH TOPOLOGY
Zigbee: topologies
147
PAN Coordinator
Full Function Device
Reduced Function Device
3 - CLUSTER TREE
PAN Coordinator
Zigbee: topologies
148
PAN Coordinator
Full Function Device
Reduced Function Device
Physical layer
Direct Sequence Spread Spectrum (DSSS)
Frequency Zone Modulation Bit-Rate Channels
149
868 Mhz
915 Mhz
2.45 Ghz
Europa
USA
Everywhere
BPSK
BPSK
O-QPSK
20 kbit/s
40 kbit/s
250 kbit/s
1
10
16
Physical layer
� Receiver Energy Detection (Free channels search)
� Scanning (Search for Beacon frames)
� Channel selection
� Clear Channel Assessment (channel
150
� Clear Channel Assessment (channel busy/available)
� Link Quality Detection (LQI: estimation of channel quality)
� Quality feedbacks to upper layers
Physical layer
4 Byte 1 Byte 1 Byte Variabile
START
151
PREAMBLE
START
of
FRAME
DELIMITER
(SFD)
Synchronization Header (SHR)
FRAME
LENGHT
(7bit)
Reserved
(1 bit)
Protocol Header (PHR)
PSDU
Payload
MAC layer: overview
� Two operation modes are defined:
� Beacon Enabled (slotted CSMA/CA)
� Non Beacon Enabled (unslotted
152
� Non Beacon Enabled (unslotted
CSMA/CA)
MAC layer: overview
� Beacon Enabled (slotted CSMA/CA)
GTS GTS Inactive
Beacon Beacon
CAP CFP
153
0 1 2 3 4 5 6 7 8 910 11 12 13 1514
Superframe Duration(SD)=aBaseSuperframeDuration*2SO simboli
Beacon Interval (BI) = aBaseSuperframeDuration*2BO simboli
� Frame duration: from 15ms to 252sec (15.38ms*2n where 0 ≤ n ≤ 14)
� Guaranteed Time Slots assigned in the beacon frame
Slotted CSMA/CA
� The time unit adopted is the backoff period (BP), usually equal to 20 symbols
� Three variables are adopted:� NB, number of access attempts for a packet� CW, number of free BPs to wait at the end of the backoff
period before starting a transmission� BE, exponent that defines the maximum number of BPs
required before starting the CCA (Clear Channel Assessment)
154
required before starting the CCA (Clear Channel Assessment) procedure
� Data transmission (and optionally the ack) must end within the CAP
� In case this is not possible, MAC has to suspend the random backoff and wait for the beginning of next CAP
� If the macBattLifeExt bit is set to 1, countdown of the backoff can be executed onliy during the first 6 BPs following the beacon
Unslotted CSMA/CA
�Classical access scheme CSMA/CA (data - ACK) without synchronization
155
MAC layer: functionalities
� Beacon Management (Synchronization)
� Channel access management
� Guaranteed Time Slot (GTS) Management
156
Management
� Association and disassociation
� Frame Acknowledgement
MAC layer: frame format
2 Byte
FRAME
CONTROL
1 Byte
SEQUENCE
NUMBER
2 Byte
FCS
variable
FRAME
PAYLOAD
0/2 0/2/8 0/2 0/2/8
Destination
PAN
Identifier
Destination
Address
Source
PAN
Identifier
Source
Address
157
CONTROL NUMBER
Codice CRC
Address fields
MAC Header
PAYLOAD
MAC Payload
IdentifierAddress
Identifier
Identify frame type address type, security, etc.
Address can be long (48 bit, IEEE) or short (16 bit, assigned by PAN coordinator)
Data Frame
158
� Up to 104 bytes payload
Ack Frame
159
� Transmitted right after data frame
Command Frame
160
� It allows the remote configuration and control of client devices
� It basically allows the implementation of a centralized network management and control for large size networks
Beacon Frame format
161
� Frame synchronization and GTS assignment
Network formation
� An FFD device search for a free channel and select a PANid (Channel Scanning). Then it starts transmitting Beacon frames
� A device that want to associate to a network scans channel and listen to Beacon frames
� Once scan is completed, a network is selected setting the access parameters according to the information in
162
the access parameters according to the information in the beacon frame
� Association is performed issuing an Associate Request Command to PAN Coordinator
� PAN Coordinator replies with a Association Response Command
Network layer: frame format
2 Byte 2 Byte 2 Byte 1 Byte 1 Byte Variabile
FRAME
PAYLOAD
FRAME
CONTROL
Destination
Address
Source
Address
Broadcast
Radius
Broadcast
Sequence
Number
Short MAC address
163
Campi di Routing
PAYLOAD
NWK PayloadNWK Header
CONTROL Number
Frame type, version, route discovery, etc.
Maximum number of hops that a message can cross (like TTL in IP)
Zigbee Routing: overview
� Defined by Zigbee Specification, published by the Zigbee Alliance (7/2005)
� Three types of devices:� ZB Coordinator (FFD)
� ZB Router (FFD)
� ZB End-Device (RFD o FFD)
164
� ZB End-Device (RFD o FFD)
� Routing is “zigbee oriented” and considers the two types of physical device (FFD RFD)
� Routing used two algorithms:� Ad-hoc On-demand Distance Vector (AODV)
� Cluster Tree Algorithm
Ad-hoc On-demand Distance Vector
� Simple on-demand routing protocol
� See part on ad hoc networks (…)
165
Cluster Tree Algorithm: tree creation
� Procedure is started from a FFD that acts as network coordinator
� Network coordinator selects one of the available channels (MAC function)
� It selects a PANidentifier to the network and assign to itself the Network Address “0” (Coordinator)
166
itself the Network Address “0” (Coordinator)
� The other devices can now join the network associating with the Network Coordinator. They can act as ZB Router(FFD) or ZB End-Device
� Once connected, ZB Routers can then allow other devices to join the network
� Address assignment is completely distributed and hierarchical
Cluster Tree Algorithm: tree creation
max
Dep
th (
L m)
Depth=0
Depth = d-1
� Hierarchical addressing simplifies routing in the tree
� Routes along the Cluster Tree Algorithm: tree creationCluster Tree Algorithm: tree creation
167
max
Dep
th (
L
nwkMaxRouters (Rm)
nwkMaxChildren (Cm)
Depth = d
� Routes along the tree or along paths created with AODV can be used based on needs
Cluster Tree Algorithm: tree creationCluster Tree Algorithm: tree creation